This invention relates to refractories and particularly refractories adapted for application by the gunning technique.
Metallurgical furnaces such as electric arc furnaces and basic oxygen vessels have utilized basic refractories having high magnesia (MgO) content. Hot repair of the refractories of such basic metallurgical furnaces is commonly done by gunning wet spray mix refractory compositions high in magnesia grain content onto the surfaces to be patched. Such refractory compositions require a binder in order to help the material set up. Various prior art binders have included phosphates, clay and water-soluble alkali metal silicates.
A common prior art binder is commonly supplied as solid particles of water-soluble sodium silicate in the hydrated form. Because such silicate includes chemically-bound water, i.e., water of crystallization, it dissolves in water very quickly.
Another water-soluble silicate binder commonly used is solid sodium silicate in the anhydrous form, i.e., without water of crystallization.
A common gunning practice is to force the refractory composition through a gunning apparatus nozzle and mix the composition with water at the nozzle to form a wet spray mix, which deposits on the refractory wall being repaired. This method is referred to as "dry-nozzle gunning".
One requirement of refractory gunning compositions is that the spray mix must adhere to the wall with minimal "rebound". A second requirement is that the spray mix must set up quite rapidly to a self-supporting condition. In other words, the wet spray mix as it is placed on the furnace wall must not be so plastic as to fall off or "slump" under its own weight. We believe that the tendency of the spray mix to slump is related to a decrease in the viscosity of the wet spray mix within the first several minutes after its deposition on the furnace wall. Therefore, one method of evaluating the tendency of a spray mix composition to slump is to evaluate its viscosity by well-known means.
It has long been recognized in the art that used linings from basic metallurgical furnaces, such as basic oxygen vessels, provide an inexpensive source of magnesia grain for gunning compositions. However, such used refractory bricks are usually discarded and have not been widely used as a source of gunning composition magnesia grain because of the lack of a good binder system.
Therefore, there is a need for a binder which can be added to crushed, used refractory bricks from basic oxygen vessels to provide a gunning composition which can be formed into a wet spray mix with the following properties:
1. MINIMAL REBOUND;
2. GOOD LOW TEMPERATURE STRENGTH;
3. GOOD HIGH TEMPERATURE STRENGTH; AND
4. NO SLUMPING.
One measure of strength of a refractory spray mix at various temperatures, as is well known, is the characteristic referred to as modulus of rupture (MOR). A good gunning composition should result in a spray mix with high modulus of rupture at low temperature, high modulus of rupture at high temperature, and good adherence characteristics.